Need help figuring out how to fix this non-manifold 3d print
The Face on the side of your mesh shows a N-Gon (Face with more than 4 Vertices). You should make sure that you only use quads (and tris if necessary) to get a good edgeflow to work with.

See if you can fix that by Hand (K is shortcut for knifetool, so you can insert edges manually) and when you're done you can also use Blenders Cleanup Function. Go to Edit mode, select all faces and go to Mesh (in the 3D viewport header) -> Cleanup. There you'll find a few options to cleanup other nonmanifold geometry like lose edges or double faces. It has Tooltips if you hover your mouse over it.

Hope this helps you, Happy Modeling :)